Tom Ridge in studio
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Ex-Homeland Security boss Tom Ridge warns of new 'grassroots terrorists'

The former director of Homeland Security Tom Ridge has told Katty Kay on the BBC's World News America programme that the Boston bombings may have been carried out by a new kind of "grassroots terrorist".

Mr Ridge said that the two suspects, Dzhokhar and Tamerlan Tsarnaev, had shown no sign anti-Americanism according to accounts given by numerous former classmates of the brothers.

He said investigators will try to determine whether the apparently normal lives of the brothers was simply a "charade" and at what point they may have become radicalised.